Cancer drug discovery is one of the most rapidly changing areas of pharmaceutical research. Most anticancer agents that are approved for clinical use are molecules which damage deoxyribonucleic acid (DNA), block DNA synthesis indirectly through inhibition of nucleic acid precursor biosynthesis or disrupt hormonal stimulation of cell growth (Sielecki, T. M. et al. J. Med. Chem. 2000, 43 (1), 1-18). There has been a recent shift of emphasis towards novel mechanistic targets that has emerged as a direct consequence of the intense study of the underlying genetic changes associated with the cancerous state. The high frequency of mutations in cancer cells which results in altered cell cycle regulation, in conjunction with aberrant expression of cyclin dependent kinases (CDKs) and growth signal transduction, conferring a proliferative advantage, indicates that many of these aberrant mechanisms may be strategic targets for cancer therapy. An increasing body of evidence has shown a link between tumor development and CDK related malfunctions. Over expression of the cyclic regulatory protein and subsequent kinase hyperactivity have been linked to several types of cancers. The process of cell division has been amply studied but the molecular mechanisms that regulate the cell cycle have only been elucidate in the last two decades. The phases of the cell cycle are: The rest phase, Go, active protein synthesis in preparation of cell division occurs in the G1 phase. During the G1 phase the volume of the cell increases. After the G1 phase the cells enter the S phase in which the DNA is replicated. The S phase is followed by another gap phase, G2, during which DNA replication is completed. The last phase is the mitosis or M phase in which the cells divide (Muhtasib, H. G. et al. Curr. Cancer Drug Targets 2002, 2, 309-336).
Rapamycin (Sirolimus, Rapamune, 1,18-dihydroxy-12-[2(4-hydroxy-3-methoxy-cyclohexyl)-1-methyl-ethyl]-19,30-4-aza-tricyclo[30.3.1% 4,9 &]hexatriaconta-16,24,26,28-tetraene-2,3,10,14,20-pentaone) with a molecular formula of C51H79NO13 and molecular mass of 913.6 Da was isolated in 1975 from the bacteria strain Streptomyces hygroscopicus found in a soil sample on Ester Island (Sehgal, S. N. et al. J. Antibiot. 1975, 28, 721 and Sehgal, S. N. et al. J. Antibiot. 1975, 28, 727). Rapamycin has potent antimicrobial, immunosuppressant and antitumor properties. It inhibits the translation of key mRNAs of proteins required for the cell cycle progression from G1 to S phase by binding intracellularly to the immunophilin FK506 binding protein FKBP12 and the resultant complex inhibits the protein kinase activity of a protein kinase termed mammalian target of rapamycin (mTOR). The inhibition of mTOR, in turn blocks signals to two separate downstream pathways which control the translation of specific mRNA (40S ribosomal protein S6 kinase p70S6K) required for cell cycle traverse from G1 to S phase (Wiederrecht, G. J. et al. Prog. Cell. Cycle. Res. 1995, 1, 53-71).
The poor aqueous solubility and chemical stability of rapamycin precluded its clinical development as an anticancer agent. Recently a series of rapamycin analogs with improved aqueous solubility and stability have been synthesized and evaluated. CCI-779 (Wyeth Ayerst, Pa., USA), a soluble ester analog of rapamycin is selected for development as an anti cancer agent based on its prominent antitumor profile and favourable pharmaceutical and toxicological characterstics in preclinical studies (Huang,. S. et al. Curr. Opin. Investig. Drugs 2002, 3, 295-304). CCI-779 has demonstrated significant inhibitory effects both in vivo and in vitro (various cell lines lines with IC50 values of <10−8 M). Its cytostatic properties results from the inhibition of translation of several key proteins that regulate the G1 phase of the cell cycle. Similar to rapamycin, CCI-779 is hypothesized to form a complex with the intracellular cytoplasmic protein FK506 binding protein −12 (FKBP) that binds to mTOR resulting in the inhibition of key signaling pathways involved in the G1 phase of the cell cycle and thereby checks the progression from G1 to S phase. Studies have shown that CCI-779 is able to penetrate the blood brain barrier as it has aqueous solubility and is highly lipophilic. Phase I and II studies have shown that CCI-779 is associated predominantly with skin toxicities (rash, folliculitis, prurtis, ulceration and nail changes), stomatic and asthenia (Elit, L. Curr Opin. Investig. Drugs 2002, 3, 1249-1253 and Punt, C. J. A. et al. Annals of Oncology 2003, 14, 931-937).
The CDK complex activity. is regulated by mechanisms such as stimulatory or inhibitory phosphorylations as well as the synthesis and degradation of the kinase and cyclin subunits themselves. Recently a link has been established between the regulation of the activity of the cyclin dependent kinases and cancer by the discovery of a group of CDK inhibitors including p27Kip1, p21Waf1/Cip1 and p16Ink4/MTS1. The inhibitory activity of p27Kip1 is induced by the negative growth factor TGF-β and by contact inhibition (Nurse et al. Nature 1994, 372 (8), 570-573). The interleukin-2 (IL-2) allows CDK activation by causing the elimination of the CDK inhibitor protein p27Kip1, which effect is prevented by rapamycin. By contrast, the CDK inhibitor p21 is induced by IL-2 and this induction is blocked by rapamycin. The activity of p21Waf1/Cip1 is regulated transcriptionally by DNA damage through the induction of p53, senesence and quiesence. The tumor suppressor protein p21Waf1 plays a central role in regulating eukaryotic cell-cycle progression. Through its association with G1 and S phase CDK complexes it regulates activation of the retinoblastoma protein (pRb) and E2F transcription factors. Thus, selective blockade of the cyclin recruitment site would prevent recognition and subsequent phosphorylation of CDK substrates, and therefore offers a therapeutic approach towards restoration of p21Waf1 like tumor suppression. Recently the octapeptide, HSKRRLIF, located C-terminal in p21Waf1 which has been shown to display potent cyclic inhibitory activity due to its capacity to bind to the cyclic recruitment site. These proteins p27Kip1, p21Waf1/Cip1 and p16Ink4/MTS1, when bound to CDK complexes, inhibit their kinase activity, thereby inhibiting progression through the cell cycle (Chen, Y. P. et al. Proc. Natl. Acad. Sci. USA 1999, 96, 4325-29; Zheleva, D. I. et al. J. Peptide Res. 2002, 60, 257-270; Atkinson, G. E. et al. Bioorg. Med. Chem. Lett. 2002, 12, 2501-2505; McInnes, C. et al. Curr. Med. Chem. -Anticancer Agents 2003, 3, 57-69.
There is therefore a need for compounds that can target the function of cell cycle suppressors such as p27Kip1 and p21Waf1/Cip1.